.container {
  height: 130px;
}
.box_5 {
  width: 100px;
  height: 100px;
  border-radius: 20px;
  border: 1px solid black;
  animation: bbr_radius 5s infinite;
}
@keyframes bbr_radius {
  50% {
    border-bottom-right-radius: 50px;
  }
}